Two of the dead were crane operators, and two were in cars crashed with the crane, said Seattle Fire Chief Harold Scoggins at a press conference.
The victims included three men and one woman, said Fire Department spokesman Lance Garland. Garland said three other people, including a four-month-old, were taken to hospital for non-life-threatening injuries.
The crane smashed six cars, according to Scoggins, when he fell.
While describing the incident as a "tragic day in Seattle," Mayor Jenny Durkan said in a statement, "We also saw some miracles, including a mother and her little child whose car was hit by the crane but survived."
Deputy Chief of Police Eric Greening said Seattle Street, where the collapse occurred, is closed until Sunday night.
tweet that people "should stay away from the crime scene and allow medical personnel and investigators to do their jobs."
Inslee described the incident as a "terrible accident" and expressed his condolences to the families and friends of the victims.
The cause for the crane collapse warnings immediately clear to the mayor, said in his statement: "The establishment of how and whythis tribe has found time will be needed"